Self-oriented anisotropic structure of G-hydrogels as a delicate balance between attractive and repulsive forces

Guanine (G) hydrogels are very attractive materials made by the supramolecular organization of G-derivatives in water. In this paper, hydrogels composed by guanosine 5’-monophosphate (GMP) and guanosine (Gua), that make long, flexible and knotted G-quadruplexes, were investigated by Small- and Wide-angle X-ray Scattering (SAXS and WAXS) to comprehend the origin of the unique orientational properties. The SAXS intensity, analysed at a fixed scattering vector modulus Q as a function of the polar angle, allowed us to derive the Maier-Saupe orientation parameter m. The strong dependence of m on hydrogel composition and temperature demonstrated that the preferred orientation is controlled by the quadruplex surface charge and flexibility. Indeed, a possible correlation between the orientation parameter m and the quadruplex-to-quadruplex lateral interactions was explored. Results confirmed that the balance between attractive and repulsive interactions plays a main role in the orientational anisotropy: quadruplex clusters lose their orientational property when attractive interactions decrease. The key role of the number of negative charges-per-unit-length of the G-quadruplex filaments was confirmed by Atomic Force Microscopy (AFM) observations. Indeed, directionality histograms showed that in the presence of large amount of Gua, G-quadruplexes follow other preferential orientations than those related to the strong interactions with the K+ pattern on the mica surface. The fact that lateral quadruplex-to-quadruplex interactions, even in the presence of external (opposing) forces, can tune the hydrogel alignment in a given preferred direction opens novel possibilities for scaffold/3D printing applications.